If you are constructing a retaining wall that is higher than two feet, there are some things you should do to ensure that it does not shift.

The construction of a retaining wall is very dependent on the foundation that the wall is sitting on.
In some areas a low retaining wall can be built directly on the ground, in other areas a concrete footing should be installed.
If the ground soil has been disturbed over the last few years, or is the type of soil that compacts easily a footing should be installed to prevent the retaining wall from sinking.

In all cases, drainage of some sort should be installed on the inside base of the wall.
Water laden soil can be twice as heavy as dry soil.
The water moves down through the soil looking for an exit route, as it is saturates the soil it will apply a tremendous amount of pressure on the backside of the retaining wall.
As there is nothing on the front side of the wall to apply pressure, the wall may begin to move forward especially near the top.

Drainage can be installed using perforated drainage pipe running perpendicular to the wall, installed at the inside base.
If you use drainage pipe, the water that has saturated the soil will eventually find its way to the pipe and be directed away from the wall. The important point here is to have an exit for that water.
This can be done by connecting to the home drainage system or by providing a 90° exit, through the retaining wall at the lowest point. When installing the drainage pipe place a layer of crushed stone all around the pipe to a thickness of at least one inch, two is even better. This prevents the holes in the drainage pipe from becoming clogged with soil.

Providing adequate drainage is important whether you are constructing your retaining wall from pressure treated timber, railway sleepers, stone or even if you are using cast concrete blocks that interlock on an angle as they sit on top of one another.

I have built a retaining wall to the rear of my garden using cavity blocks. It is 25ft long and 5 blocks high but I have not put in any drainage. I have some rocks along the first course but no weep holes. Would it be sufficient just to drill weep holes in the wall or would you advise digging out the back fill and putting in a course of washed stone ?
Any advise would be much appreciated !!
Tom

Hello tomf. I would surely create a series of drainage or weep holes at the base of the wall, every 2 meters (6 ft).
You say the wall is 5 blocks high, does this bring it to over 1 metre (3 ft) in height? Soil retaining walls over 1 metre (3 ft) in height should really be reinforced with steel when constructing.
I presume your wall is not reinforced or you would have said it in your post. If this is the case, I would advise you to dig out the soil at least 1 ft behind the wall. Dig this soil out right to the base and replace with clean crushed stone.
When you are doing this why not consider placing a damp proof barrier behind the wall.
Good luck, let us know how you get on. with your garden drainage

Posted: Mon Aug 21, 2006 8:43 am Post subject: not sure if I want to dig out the back fill

tomf wrote:

still not sure if I want to dig out the back fill. And no I don't have any reinforced steel in it, could you foresee this being an issue ? Tom

It really depends how much soil the wall is holding back, if the soil holds a lot of water (is sticky) and how the soil was laid down (treaded layers is best).
Its hard to tell really, you may have to go with your gut on this. If I had a pic of the soil / wall / surrounding area, I may be able to help you more.

In addition to the problems listed in the first posting consider soil ice formation in wet soil. Apart from the increased weight when wet, when it subsequently freezes it can expand hugely. It can push out any weak boundary wall (and even some strong ones). Hope I'm not depressing anyone !

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